The present invention relates to a water-soluble composition for providing a temporary coating, in the form of a dry film, on metal surfaces of various forms, which are subject to atmospheric oxidation. It also relates to the method for applying the composition to these surfaces and to its use for protecting various specific metal surfaces.
After metal plates or tubes have been shaped, it is frequently necessary to store them for many months under shelter before using them. Such storage, which is always possible, is a source of concern to producers as, in the absence of protection against air and humidity oxidation, a layer of oxidation forms at the surface, or the surface is subject to pitting or other phenomena corresponding to a progressive deterioration of the surface state of these materials. Any alteration or deterioration of their surface can however have a considerable impact on their implementation or on the long-term effectiveness of subsequent treatments to which they may be subjected. This involves decreasing the risk of breakage or cracking during shaping or deep drawing of these materials or, yet again, flaking or cracking of coatings applied to them.
To avoid these disadvantages associated with atmospheric oxidation, it is necessary to keep these metal surfaces, plates or tubes away from the air, and several techniques aimed at this have already been employed.
Thus, various types of temporary coatings, impermeable to air, have been developed. Apart from their temporary character, such coatings should be easy to apply, and should also be readily eliminated without this leading to supplementary pollution, modification of the underlying surface state, or difficulties during their handling.
A first type of coating habitually employed consists of oily or semi-oily: compositions obtained by mixing water and mineral oil into an emulsion, in the presence of a corrosion inhibitor as disclosed in U.S. Pat. No. 4,342,596.
Another type of oily coating comprises an emulsified mixture in water of paraffins and partially esterified paraffins, of a small proportion of amides resulting from the reaction of an amine on a long chain carboxylic acid, or a mixture of alcohol, hydrocarbons and surfactant as disclosed in U.S. Pat. No. 4,479,981.
Such coatings are presented as having good stability and forming an excellent barrier against atmospheric oxidation. Additionally, they perform a lubricating function during subsequent treatment of the materials thus protected. Nevertheless, for simple storage, the oily character of these coatings is a source of inconvenience and not inconsiderable pollution.
Other temporary protection coatings are obtained by spreading water soluble compositions on the surface to be protected from atmospheric air, such water soluble compositions being obtained by mixing polyoxyalkyleneglycols and alkyl derivatives of succinic anhydride such as the composition disclosed in U.S. Pat. No. 5,316,696. These coatings, although they are not oily, constitute liquid protective films at the surface of the metallic materials.
Additionally, in European patent application 0,566,087 it is stated that certain combinations of mono- and dicarboxylic acids can be employed as atmospheric corrosion inhibitors in the form of coatings on materials to be protected, the nature of the coating not being defined.
The present invention aims at providing a coating for protecting metal surfaces from atmospheric corrosion, taking the form of a non-oily dry film allowing easier handling of these surfaces while simultaneously limiting the risk of polluting the premises where they are stored or of dirtying the hands of people handling them. It also sets out to provide a dry film which is readily eliminated by washing with water and drying, or which can remain on the metal surface beneath a subsequently applied coating, paint layer or other surface coating. For certain paints and certain varnishes, the presence of the dry film improves adhesion of subsequent layers.
The present invention consequently provides a water-soluble composition as a temporary coating, in the form of a dry film, for metal surfaces subject to atmospheric oxidation, containing at least one carboxylic acid, characterised in that it comprises at least 20% by weight of a combination of at least one saturated linear carboxylic monoacid with at least one unsaturated linear carboxylic monoacid, in the form of alkaline, alkaline-earth or amine salts, the carbon chain of each acid comprising an uneven number of carbon atoms varying from 5 to 21 and, optionally, at least 0.5% by weight of a triazole derivative.
The combination of two uneven-numbered carbon chain carboxylic monoacids, one being saturated and the other unsaturated brings about, in a dry film, a completely unexpected improvement in the effectiveness of protection, notably against the formation of corrosion on the metal surfaces. The atmospheric anti-corrosion effect of this synergy is enhanced, for zinc-coated steel, by the addition of a triazole compound.
More precisely, this composition according to the invention comprises from 25 to 40% by weight of said combination, from 0.5 to 5% by weight of at least one triazole derivative, and from 40 to 50% by weight water.
In a preferred embodiment, the combination is constituted by two carboxylic monoacids selected from saturated acids comprising 5 to 15 carbon atoms and from unsaturated acids comprising from 9 to 21 carbon atoms, these two acids having a saturated acid/unsaturated acid molar ratio comprised between 5 and 20.
In particular, the combination is constituted by from 20 to 35% by weight of the composition of at least one half of a saturated acid selected from heptanoic acid and nonanoic acid and from 2 to 5% by composition weight of an undecylenic acid salt.
The triazole derivative according to the invention is selected from the group constituted by benzotriazoles, tolyltriazoles and alkyltriazoles.
To facilitate covering of the metal surface to be protected and to accelerate drying, preferably from 0.005 to 2% by weight of at least one wetting agent is added to the composition. This wetting agent is chosen from the group comprising anionic surfactants such as alkyl sulfates or non-ionic. surfactants such as ethoxylated compounds of the fatty alcohol or alkylphenol type.
The invention also provides a method for depositing and forming a temporary protective film from said concentrated composition, consisting in:
diluting said composition to between 1 and 10% by weight in water:
bringing the solution thus formed into contact with the metal surface,
draining said, solution and then delivering a stream of air, optionally heated, to said metal surface until it is dry.
In order to bring the solution formed into contact with the metal surface, any means, notably sprinkling, spraying or, yet again, immersion can be employed.
The film thus obtained takes the form of a dry, non-oily, film which is not sticky and which leaves no trace on the hands during handling of plates or bundles of metal tubes, and which protects them from atmospheric corrosion.
To remove the film, simple washing of the metal surfaces with water followed by air drying is sufficient.
The invention thirdly covers the use of these concentrated compositions in the form of a dry film for protecting metallic surfaces in carbon steel, zinc coated or galvanised, and the copper and aluminium alloys.
In the remainder of this description, examples are provided by way of illustration which however are not limiting.